Tera in our midst: New picture emerges

Ezine

Published: Sep 15, 2014

Author: David Bradley

Channels: Atomic

Good vibrations

New research into terahertz radiation could lead to light detectors that can see below the surface of bodies, walls, and other objects, with applications in fields as diverse as mobile communications, medical imaging, chemical sensing, night vision, and security.

Xinghan Cai, Andrei Sushkov, Gregory Jenkins and Dennis Drew of the Center for Nanophysics and Advanced Materials, University of Maryland, College Park, in Maryland, Ryan Suess, Mohammad Jadidi, Shanshan Li and Thomas Murphy of the Institute for Research in Electronics and Applied Physics, physicist Luke Nyakiti of the Texas A&M, Jun Yan of the of the University of Massachusetts, Amherst, and Michael Fuhrer of Monash University, in Victoria, Australia, are supported through grants from the US Office of Naval Research, the National Science Foundation and the Intelligence Advanced Research Projects Activity.

The team has turned to graphene to develop a sensitive terahertz detector that could probe below the surface of our skin, and perhaps even see through walls. Graphene is well known as a Nobel Prize winning two-dimensional form of carbon that has some intriguing and potentially very useful opto-electronic properties. Of particular interest to Fuhrer and colleagues is the extraordinarily broad band of electromagnetic wavelengths to which a graphene-based detector could respond. This broad band includes the terahertz region of the spectrum, 1 mm to 0.1 mm wavelengths. This relatively long wavelength and low frequency range falls between the microwave and infrared part of the spectrum. As such terahertz radiation is transmitted by many materials that we normally consider to be opaque, such as skin, plastics, clothing and cardboard. It has thus been for the focus of attention for airport security imaging and medical imaging as an alternative to ionizing forms of radiation commonly used in both settings. Moreover, terahertz radiation has a characteristic spectrum for given chemical structures.

Improved detectors that operate at ambient, as opposed to cryogenic, temperatures are much needed and will open up many technological applications beyond the limited number seen today. Existing room temperature terahertz detectors are bulky and slow as well as being prohibitively expensive, in general. Graphene could change all that. The team exploited a newly identified operating principle called the "hot-electron photothermoelectric effect" to build their device, which Monash's Fuhrer says is "as sensitive as any existing room temperature detector in the terahertz range and more than a million times faster." Because graphene is a single atom thick, energy absorbed from terahertz radiation cannot be easily dissipated as heat through lattice vibrations.

Broadband penetration

As such, terahertz radiation is transmitted by many materials that we normally consider to be opaque, such as skin, plastics, clothing and cardboard. It has thus been for the focus of attention for airport security imaging and medical imaging as an alternative to ionizing forms of radiation commonly used in both settings. Moreover, terahertz radiation has a characteristic spectrum for given chemical structures.

Improved detectors that operate at ambient, as opposed to cryogenic, temperatures are much need and will open up many technological applications beyond the limited number seen today. Existing room temperature terahertz detectors are bulky and slow as well as being prohibitively expensive, in general. Graphene could change all that. The team exploited a newly identified operating principle called the "hot-electron photothermoelectric effect" to build their device, which Monash's Fuhrer says is "as sensitive as any existing room temperature detector in the terahertz range and more than a million times faster." Because graphene is a single atom thick, energy absorbed from terahertz radiation cannot be dissipated as heat through lattice vibrations.

Electrical tap

The concept behind the new detector hinges on this fact. Terahertz energy is absorbed by the electrons, which then become excited and can be tapped of by an electrical connection to the graphene layer. The prototype devices uses two electrical leads made of different metals, gold and chromium, which conduct the electrons away at different rates. This conductivity difference gives rise to the electrical signal that flags the absorption of terahertz radiation. Terahertz radiation can penetrate materials that visible light cannot reach, but only so far it does not go as deep as bone, as X-rays do, however. The speed and sensitivity of the room temperature detector presented in this research opens the door to future discoveries in this in-between zone.

"We are working on using a an effect called plasmonic resonance to enhance the absorption of THz in graphene at specific frequencies," Fuhrer told SpectroscopyNOW. "This will make the detectors more sensitive and also responsive to sepcific frequencies, and the frequency can be electrically tuned using a gate voltage. We are also working on schemes to use plasmonic resonance to make emitters of THz using graphene."